The present invention relates to fiber optic communications, and more particularly to fiber optic wavelength division multiplexers.
Wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) allow data transmission over multiple channels per fiber and thus greatly increase data transmission capacity per fiber. Typical channel spacing in DWDM systems has been progressively reduced in order to increase capacity even further. At the present time, 200 GHz, 100 GHz, and 50 GHz channel spacings are in use.
Interleavers and deinterleavers are commonly used to combine and separate channels. An interleaver receives two sets of channels at a given channel spacing and combines the two sets into one set having half the channel spacing. A deinterleaver performs the opposite function by receiving a set of channels at a given channel spacing and separating the received set into two sets of alternating channels with twice the channel spacing. Many types of interleaver and deinterleaver are known, including fused-fiber Mach-Zehnder interferometer, liquid crystals, birefringent crystals, Gires-Tournois interferometer (GTI) and others. The GTI based interleaver has many advantages including, very low insertion loss, uniform response over a wide range of wavelengths (flat-top spectrum), and minimal polarization dependence effect. However, the GTI based interleaver does have larger chromatic dispersion.
FIG. 1 shows the passband shape and characteristics of a 100/200 GHz deinterleaver. The input channel separation (spacing) is 100 GHz. The typical insertion loss is 1 dB, the maximum insertion loss is 1.5 dB. The 0.5 dB passband width is not less than 27.5 GHz. The xe2x88x9225 dB isolation rejection width is not less than 27.5 dB. The absolute value of the chromatic dispersion is not larger than 30 ps/nm. The specifications of a 100/200 GHz interleaver are the same as the deinterleaver, with the exception that in an interleaver xe2x88x9215 dB isolation is good enough.
As the required data capacity of a fiber is increased, more channels are required in a given bandwidth, and thus smaller channel spacing is required. Thus, as greater data capacity is required, interleavers and deinterleavers must function at smaller channel spacings, for example at 50/100 GHz and even at 25/50 GHz. In order to retain a high speed of data transmission of 10 Gbit/sec, the required specifications of an interleaver or deinterleaver with the narrower channel spacing are almost the same as for an interleaver or deinterleaver with wider channels spacing. As the channels spacings of interleaver or deinterleaver are reduced by half from 100/200 GHz to 50/100 GHz, the passband and stopband widths are reduced by half and the chromatic dispersion increases by a factor of four. Therefore, there exists a need for a deinterleaver with increased stopband width and with smaller chromatic dispersion.
FIG. 2 shows cascaded deinterleavers for separating input channels into four sets of output channels. Deinterleaver 22 receives a light beam 24 containing channels xcex1,xcex2,xcex3 . . . and outputs two light beams 26 and 28, light beam 26 containing channels xcex1,xcex3,xcex5 . . . and light beam 28 containing channels xcex2,xcex4,xcex6 . . . Deinterleaver 30 receives light beam 26 and outputs two light beams 32 and 34, light beam 32 containing channels xcex1,xcex5,xcex9 . . . and light beam 34 containing channels xcex3, xcex7,xcex11. Deinterleaver 36 receives light beam 28 and outputs light beams 38 and 40, light beam 38 containing channels xcex2,xcex6,xcex10 . . . and light beam 40 containing channels xcex4,xcex8,xcex12. In the example shown in FIG. 2, deinterleaver 22 is a 50/100 GHz deinterleaver and deinterleavers 30 and 36 are 100/200 GHz deinterleavers. In FIG. 2, the deinterleavers 22, 30 and 36 are separate units and the light beams 26 and 28 are normally carried by optical fibers from deinterleaver 22 to deinterleavers 30 and 36. This arrangement is expensive and excessively bulky. There is a need for compact single unit deinterleavers having 4 outputs as above. There is also a need for compact single unit interleavers for receiving four sets of channels and outputting one combined set of channels.
It is an object of the present invention to provide a deinterleaver that has high isolation and dispersion compensation.
It is an object of the present invention to provide an interleaver and deinterleaver that includes two optical filter stages to improve channel isolation and also includes dispersion compensation to compensate for chromatic dispersion introduced in the interleaver or deinterleaver.
It is also an object of the present invention to provide a dispersion compensated 1 to 4 deinterleaver.
It is also an object of the present invention to provide a dispersion compensated 4 to 1 interleaver.
The objects and advantages of the present invention are obtained in a dispersion compensated 1 to 2 deinterleaver and a dispersion compensated 2 to 1 interleaver in which the light containing signals of even channels passes through a dispersion compensator and two polarization interferometers having pass band for all channels, and the light containing signals of odd channels also passes through a dispersion compensator and two polarization interferometers having pass bands for all channels, resulting in greater stopband width and greater channel isolation.
The 1 to 2 deinterleaver includes a dispersion compensator and three polarization interferometers. Light containing signals for odd and even channels passes from a first port A through the dispersion compensator, through a first polarization interferometer, is divided by a polarization beam splitter into light containing signals of even channels and light containing signals of odd channels, the light containing even channels passing through a second polarization interferometer to a second port, and light containing the odd channels passing through a third polarization interferometer to a third port, wherein all three polarization interferometers have pass bands for odd and even channels.
The 2 to 1 interleaver uses the same optical components as does the 1 to 2 deinterleaver so that light containing signals for even channels passes from the second port through the second polarization interferometer, then through the first polarization interferometer and there combines with light containing signals of odd channels that has passed from the third port through the third polarization interferometer and the combined light containing signals for odd and even channels passes through the dispersion compensator to the first port.
The objects and advantages of the present invention are also obtained in a dispersion compensated 1 to 4 deinterleaver and a dispersion compensated 4 to 1 interleaver, in which the light containing signals of all channels passes through a dispersion compensator.